1. Field of the Invention
This invention relates in general to radar devices and in particular to a circuit arrangement for side lobe suppression in a radar device utilizing a loop as part of a compensating circuit in which an undesired signal received by way of the side lobes of the radar antenna is compensated in an adder using an auxiliary signal obtained with an omnidirectional auxiliary or phantom antenna after it has been weighted in a weighting multiplier using a weighting factor developed in a feedback branch between a correlator and the weighting multiplier of the side lobe canceller (SLC).
2. Description of the Prior Art
Undesired signals received by side lobes of a radar antenna can result in a significant degradation in the radar receiver in the processing of useful signals. This undesired result becomes particularly evident when it is desired to use small antennas in conjunction with high antenna gain in the direction of maximum radiation to result in reduced side lobe attenuation. The antenna reflector is illuminated to the greatest possible degree by the primary radiator. This results in side lobes arising to an increased degree and said side lobes make it easier for a source of unwanted signals or jamming signals to pass into the radar signal and disrupt the signal processing procedure even when the major lobe of the radar antenna is not directed toward the source of the unwanted or jamming signals. A source of unwanted or jamming signals of sufficient strength can thus substantially reduce the effectiveness of the radar over its entire azimuth detection range.
It has previously been known to selectively attenuate the noise energy by means of so-called side lobes suppression without significantly influencing the signal received from the major lobe.
One of the known circuits for side lobes suppression functions according to the principal of cancelling the noise signal and is designated as a "side lobe canceller" (SLC). In addition to the radar receiver and its antenna such prior art circuit requires a section reception branch with an auxiliary or phantom antenna which has omnidirectional characteristics. Such circuit functions in that a signal received in the side lobe range of the primary antenna is also received on the auxiliary or phantom antenna and is rotated or respectively attenuated in the second reception branch as to phase and amplitude by means of a control loop such that it is equal to the signal from the primary channel. By subtracting the two signals, it is possible to cancel the noise signal as described in the publication MIL. Microwave Conference, London 1978, Page 370.
The two reception branches are linked to each other with a loop which consists of an adder, a weighting multiplier for the auxiliary signal and a correlator which generates a weighting factor for the weighting multiplier in a feedback branch and utilizes a circuit for the mean value formation and further includes an amplifier.
Only the noise signals occurring in the side lobe range of the radar antenna are to be eliminated with such a circuit arrangement. Targets detected by the major lobe of the radar antenna in contrast are to be received with as little attenuation as possible. A possibility of reducing the effect of the loop exists in that the weighting factor for the weighting multiplier is limited to a predetermined value, for example, a value of 1. This method, however, has the disadvantage that the signal voltage of a desired target is attenuated in the ratio of the gain of the auxiliary or phantom antenna/gain of the radar major lobe. Moreover, the thermal noise of the auxiliary channel is added in full strength (given amount 1) in the range of the major lobe into the radar channel whereby the signal to noise ratio of the target signal is reduced.
When predominantly indirect reception of a noise signal in a high reflection environment occurs, the signals in the radar and auxiliary channels can be uncorrelated as a result of multi-path propagation or wide angle diffuse reflection. Since in this case, the correlation product between the radar and auxiliary signal will be zero, the weighting factor formed in the loop will under certain conditions retain its previous value. This property can have a negative influence on the output signal of the radar channel since the noise energy evaluated with the weighting factor effective at such time will be added into the radar channel.
The prior art circuit for side lobe suppression has the disadvantage that the transient response of its loop depends on the noise signal power. Usually, the loop is formed in a manner such that with low signal noise levels the drag error is still tolerable and a high noise signal level does not produce self-excitation of the loop.
Although the loop dynamics can generally be adapted to the noise level dynamics utilizing loops which function in an analog manner, the dynamics of digitally functioning loops is not sufficient in all cases due to the stability limit which depends on the scanning rate. It then becomes necessary to accept a drag or timing error or to limit the noise power. Given SLC circuits which are formed in analog technology, an automatic gain control (AGC) can be provided in the auxiliary channel which maintains the signal level constant. This control, however, deteriorates the dynamic behaviour of the circuit when amplitude-modulated noise sources or jammer sources occur. A sudden boost of the noise amplitude thus results in an output signal until the weighting signal has responded to the new value.